Upon a reduction in energy, AMPK is activated to stimulate energy producing pathways and turn off energy consuming pathways to restore the ATP/ADP ratio. Thus, AMPK is known to inhibit acetyl-CoA carboxylase, lower malonyl-CoA and increase the activity of carnitine palmitoyltransferase-1 to facilitate FA uptake and oxidation in the mitochondria [25, 26]. Stimulation of AMPK is also known to modulate FA uptake via CD36 [27]. Our laboratory has extensively published on the role of AMPK in LPL translocation to the vascular lumen [28]. Specifically, we reported that AMPK plays a role in vesicular formation and subsequent movement of LPL along the actin cytoskeleton in cardiomyocyte through activation of heat shock protein 25 [2]. Thus, physiological and pathological processes that change AMPK are known to impact coronary LPL activity. For instance, following overnight fasting, AMPK is activated to increase coronary LPL, that guarantees FA delivery to meet the energy demand in this nutrient deficient condition [29]. Using streptozotocin (STZ) to induce moderate diabetes in rats, we reported that like fasting, acute hypoinsulinemia stimulated AMPK phosphorylation, and resulted in an augmented coronary LPL activity. This enabled the heart to switch its substrate utilization to exclusively using LPL-derived FA [10]. With a higher dose of STZ to induce severe diabetes, these animals developed both hyperglycemia and severe hyperlipidemia with increased circulating FA [30], that are known to inhibit AMPK activation [31]. In hearts from these animals, LPL activity was reduced and an unregulated uptake of NEFA resulted in cardiac lipotoxicity and dysfunction [30, 32]. Overall, our data suggested that activation of AMPK is a significant contributor towards LPL movement and subsequent FA utilization. Thus, agents that are capable of increasing cardiac LPL activity through the AMPK pathway may be useful for preventing NEFA uptake and lipotoxicity following diabetes.

Hpa is an endo-β-glucuronidase that is produced in EC as an inactive latent protein (Hpa^L). Following its synthesis, it is secreted to be taken up by HSPG and stored in lysosomes [33, 34]. At this location, enzyme processing results in a 50-kDa polypeptide that is significantly more active (Hpa^A) than Hpa^L [35, 36]. Both forms of Hpa are stored within the EC until secreted in response to various stimuli. Related to its physiological functioning, Hpa has roles in embryonic development, wound healing and hair growth [37]. Studies from our lab was the first to identify a novel role of Hpa in cardiac metabolism. In this regard, we described how Hpa released cardiomyocyte LPL for subsequent transfer to the vascular lumen for FA generation [38]. In people living with Type 2 diabetes, plasma and urine levels of Hpa are increased [39, 40]. In vitro studies using EC established that acute incubation of these cells with high glucose had a robust influence on Hpa secretion [41]. Using an animal model of STZ-induced diabetes, isolated hearts released significantly higher amounts of both forms of Hpa within the first 5 min, with Hpa^L secretion being greater than Hpa^A [42]. Related to Hpa^A, its heparan sulfate hydrolyzing ability would be capable of releasing myocyte surface-bound proteins including LPL. Intriguingly, enzymatically inactive Hpa^L is also able to initiate HSPG-clustering that activates p38 MAPK, Src, PI3K-Akt, and RhoA [38, 43–46]. These signalling pathways could then allow for replenishment of the cardiomyocyte pool of LPL that was released by Hpa^A. Overall, circulating Hpa has an important role in the communication between EC and cardiomyocytes to eventually supply FA to the heart. Intriguingly, unlike high glucose, when EC are exposed to increasing concentrations of palmitic acid, the nuclear content of Hpa was augmented [41]. Moreover, in recently published data from our lab, severe diabetes with concomitant hyperglycemia and hyperlipidemia reduced Hpa secretion from the isolated heart, a possible explanation for the lowered coronary LPL activity in these hearts [47]. Currently, whether manipulation of EC Hpa is capable of influencing cardiac metabolism following diabetes is unknown and should be investigated.

To determine the contribution of HSPG to LPL transcytosis, LPL accumulation was determined following knock-out of GIPHBP1. In this condition, LPL in skeletal muscle and heart collected more at the basolateral side of EC as compared to the cardiomyocyte side suggesting that an HSPG gradient determines the direction of LPL flow from the underlying cardiomyocyte to the basolateral surface of EC [48]. At this location, collagen XVIII also acts as a reservoir for LPL [49].

LPL is expressed mainly in parenchymal cells like cardiomyocytes, whereas GPIHBP1 is located exclusively in capillary ECs. Interestingly, a comparable distribution of GPIHBP1 is described at both the luminal or abluminal sides of these cells [16, 50]. It should be noted that ECs that are part of large blood vessels, arterioles and venules do not display GPIHBP1 [16]. Regarding the binding of LPL to GPIHBP1, this occurs at a 1:1 ratio and with a higher affinity when compared to its binding to HSPG [51]. Structurally, as GPIHBP1 contains a GPI anchor, its release from the plasma membrane is achievable with phosphatidylinositol-specific phospholipase C that is known to digest this anchor [14]. It is the acidic domain of GPIHBP1 that can ionically attach LPL [52]. Given its defined role in the bidirectional translocation of LPL across the EC [16], and its ability to serve as a platform to promote lipoprotein-TG hydrolysis (it allows lipoproteins to stay bound/marginate to heart capillaries for several minutes [53, 54]), its absence in GPIHBP1 knockout mice causes robust hypertriglyceridemia even when these animals are fed a low-fat diet [16]. Similar effects are seen in patients with GPIHBP1 mutations [55]. More recent functions of GPIHBP1 include its ability to prevent the unfolding of LPL by angiopoietin-like protein 4 (ANGPTL4) [56–58]. Regarding its regulation, GPIHBP1 expression can be affected by fasting/refeeding [59]. Fasting augments cardiac GPIHBP1, and this effect can be overcome by refeeding [59]. Following diabetes, cardiac GPIHBP1 gene and protein expression also increase with an associated augmentation of coronary LPL activity [60]. Moreover, in vitro incubation of EC with high glucose also caused a rapid increase in GPIHBP1 mRNA and protein [50, 61]. Intriguingly, exposure of EC to Hpa^L or Hpa^A produced a significant increase in GPIHBP1 gene and protein [50]. Given that high glucose can stimulate the secretion of both forms of Hpa, this could be one mechanism by which the EC can increase GPIHBP1 to accelerate FA delivery to the cardiomyocytes after diabetes.

ANGPTL 3, 4, and 8 are endogenous LPL antagonists. ANGPTL3 is exclusively expressed in the liver whereas ANGPTL4 and 8 are abundant in the liver, adipose tissue and muscle. One way by which fasting decreases LPL in the adipose tissue is that nutritional deprivation increases ANGPTL4 in adipose tissue. This is a positive effect as circulating TG are then diverted towards oxidative tissues for provision of energy [62].

FA is known to affect LPL in multiple ways, including a) FA inhibition of LPL movement in the cardiomyocyte [63], b) FA suppression of Hpa secretion, thus reducing cardiomyocyte to EC transfer of LPL [41], c) FA detachment of vascular LPL for hepatic degradation [64], and d) FA inactivation of LPL, either directly [65] or through induction of ANGPTL4 [66–68]. With severe diabetes, animals developed hyperlipidemia that was associated with a reduction in heparin-releasable LPL activity in the heart [32]. This occurred in the absence of any change in LPL gene expression [69] suggesting that following diabetes, cardiac LPL activity is mainly modulated by post-translational mechanisms. In this regard, when RNA-seq was performed in diabetic hearts, of the more than fifteen hundred differentially expressed genes, the one that showed the greatest fold change (∼25-fold increase) was ANGPTL4 [30]. Altogether, these results imply that circulating FA has the ability to supress vascular LPL by a host of mechanisms to prevent lipid overload of the heart.

Changes in circulating insulin can affect LPL and this response varies with the tissues being studied [70]. Thus, a reduction in insulin after fasting decreases adipocyte LPL but enhances its activity in the heart [71], changes that occurred in the absence of LPL gene or total protein expression [29]. Consequently, the FA that are produced from lipoprotein-TG lipolysis by LPL are directed away from storage in the adipose tissue so that they can fulfill the metabolic demands of cardiomyocytes. As newly synthesized LPL can transfer from myocytes to the vascular EC within 30 min, an augmented vectorial movement of LPL could explain the rapid increase of coronary LPL following fasting [72]. Mechanistically, a reduction in insulin after fasting or STZ-induced diabetes decreases glucose uptake in the heart resulting in activation of AMPK [1, 73, 74] with stimulation of LPL translocation [2] (see detailed review [19]).

Activators of LPL include Apolipoproteins (Apo) C-II and Apo A-V, whereas inhibitors include Apo C-III. Apo-CII is produced primarily in the liver and then incorporated into lipoproteins. On binding to LPL, it promotes conformational changes in the enzyme, allowing the catalytic site of LPL to interact with lipoproteins permitting their hydrolysis [75]. Apo A-V increases the activity of Apo C-II [76, 77].

Given the contribution of LPL in supplying FAs to various tissues for storage and energy generation, in addition to its role in plasma lipoprotein clearance, the action of LPL is considered beneficial. Thus, the contribution of LPL towards the etiology of atherosclerosis is contentious. On the one hand, overexpression of LPL has been shown to protect against diet-induced atherosclerosis in Ldlr^−/− and Apoe^−/− mice, established animal models to study atherosclerosis [99, 100]. This protective effect of LPL was linked to beneficial changes in plasma lipoproteins. On the other hand, this was not the case when LPL levels were manipulated in macrophages. Specific deletion of macrophage LPL (with no changes in total plasma LPL activity) significantly reduced atherosclerotic lesions in Apoe^−/− mice, supporting an important role for macrophage LPL in atherosclerosis [101]. Similarly, overexpression of human LPL in rabbit macrophages accelerated atherosclerotic plaque development, and this occurred in the absence of any changes in plasma lipids [102]. These studies reinforced a pro-atherogenic role for macrophage LPL. It is worth noting that although LPL has abundant expression in highly oxidative tissues, it is also detected in tissues like the kidneys, brain and macrophages where its binding properties are likely more important than its lipolytic activity [103]. Thus, the effect of macrophage LPL on atherosclerosis could be the result of its lipolytic activity and/or its ability to act as a receptor to assist remnant particle uptake. Related to the latter function of LPL, subsequent to lipolysis of lipoproteins, the remnant particles that detach from the endothelial GPIHBP1 platform have inactive LPL bound to them. It has been proposed that this inactive LPL may act as a hepatic receptor ligand to promote lipoprotein uptake [104–106]. When this process occurs in macrophages, LPL acted as a bridging molecule for remnant uptake to increase foam cell formation and lesion progression (Figure 1) [107].

The above discussion brings forward the contribution of macrophages towards foam cell formation and development of atherosclerosis. However, more recent studies have also implicated SMCs in foam cell formation, at least in humans [108]. Thus, in mouse models of atherosclerosis, the lipid milieu along with the resident inflammation recruits’ monocytes across EC to initiate atherosclerosis [109]. In contrast, in human atherosclerosis, SMCs migration from the arterial media occurs before lipid accumulation and in fact SMC make up almost 50% of the foam cells in the plaque lesion [108]. It is possible that SMCs take up lipoprotein remnants through expression of scavenger receptors. However, LPL expression in SMC may also contribute to this mechanism. In vitro, SMC and macrophages synthesize LPL [110] which could act as a co-receptor to facilitate the binding of native and ox-LDL to HSPG (Figure 1) [111]. Interestingly, LPL has been detected in the fibrous cap of the atherosclerotic lesions [112]. Whether SMCs in the atherosclerotic lesion synthesize LPL per se or whether it is transferred from the macrophages is currently unknown. It should be noted that at least in the heart, the translocation of LPL has been reported from cells like the cardiomyocyte to endothelial cells [86]. In support, SMC interact with macrophages both directly and indirectly. For example, in the presence of macrophages, SMC increases their phagocytic activity by enhancing LPL and proteoglycans to promote lipoprotein uptake [113].

Oral glucose causes the release of gut hormones like glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) that amplify glucose-induced insulin secretion in addition to acting via multiple mechanisms to influence blood glucose [114]. Drugs that mimic GLP-1 (e.g., Semaglutide) and/or GIP (e.g., Tirzepatide) are gaining in popularity not only due to their control of blood glucose but also due to their ability to promote weight loss [115, 116]. Intriguingly, cardiovascular outcome trials demonstrate that long-term use of GLP-1 receptor agonists reduce cardiovascular complications of diabetes [117] or obesity [118] by mechanisms that not completely understood. It has been proposed that incretins could offer cardioprotection via their influence on lipid metabolism. In this regard, GLP-1 has been shown to regulate secretion of lipoproteins and cholesterol metabolism [119, 120]. GIP, on the other hand, demonstrated a regulatory role in lipid metabolism that occurred partially via LPL activation. Thus, in cultured 3T3-L1 cells and human adipocytes, GIP stimulated LPL activity in a dose-dependent manner [121–123]. These results were supported by in vivo studies which reported that GIP accelerated chylomicron TG clearance in dogs [124]. Similarly, human studies revealed that GIP infusion significantly increased LPL action, where LPL-derived FA largely contributed to an increase in re-esterification rate and TG storage in adipose tissue of lean individuals [125]. However, unlike adipose tissue, the effects of GIP on cardiac LPL level remain unclear. Given the opposing mechanisms of LPL regulation between adipose tissue and the heart, it is possible that GIP lowers cardiac LPL activity. Interestingly, eliminating GIP receptor signaling protected the heart against experimental myocardial infarction, and this was associated with reduced phosphorylation of HSL and increased cardiac TG storage [126]. As intramuscular TG accumulation is predominantly regulated by the action of HSL and LPL [127], the contribution of cardiac LPL to the altered lipid accumulation and the cardioprotective phenotype of Gipr−/− mice following myocardial infarction would be interesting to study.

Apolipoprotein C3 (APOC3) and angiopoietin-like protein (e.g. ANGPTL3) are known to inhibit LPL activity directly. Thus, a genetic reduction in APOC3 increases LPL activity, reduces plasma TG and causes a decrease in coronary heart disease [128]. Related to targeting APOC3, two antisense oligonucleotides, Olezarsen and Volanesorsen, are currently in Phase 3 clinical trials to determine their therapeutic potential to lower circulating TG [129, 130]. Similar to APOC3, genetic variants in ANGPTL3 impacts plasma TG [131]. As such, Evinacumab (human monoclonal antibody) is currently on the market to inhibit the action of APOC3 to lower TG levels [132].

Used in statin-treated patients with elevated TG (≥150 mg/dL), who are at high risk of cardiovascular events due to established cardiovascular disease, or diabetes, and at least one other cardiovascular risk factor. Mechanism of action not completely understood and likely multifactorial and includes a decreased production and accelerated clearance of triglycerides [133]. It decreases VLDL synthesis and secretion by a) reducing hepatic lipogenesis (synthesis of FA from acetyl CoA), b) increasing beta-oxidation of FA and c) inhibiting TG-synthesizing enzymes (e.g., DGAT). It increases VLDL clearance by a) augmenting LPL activity directly or b) indirectly by reducing APOC3, an inhibitor of LPL. It’s a unique form of omega-3 fatty acid (eicosapentaenoic acid) that reduces VLDL-TG. Affects multiple atherosclerotic processes including endothelial function, oxidative stress, foam cell formation, inflammatory response (its anti-inflammatory), platelet aggregation and plaque rupture (it causes plaque regression) [134, 135].

Also called fibrates (e.g., fenofibrate and bezafibrate). Function primarily as peroxisome proliferator-activated receptors (PPAR) agonists (stimulates the PPARα receptor), thereby increasing the oxidation of fatty acids in liver (decreases VLDL production) and striated muscle (also kidney and heart). Also increases LPL activity (increased catabolism of circulating TGs increases the rate of clearance of TG) by both transcription upregulation of LPL and down regulation of APOC3 (an inhibitor of LPL) [136, 137].

They are reversible, competitive inhibitors of HMG-CoA reductase. As a result, there is inhibition of intracellular cholesterol synthesis mainly in the liver. Because a precise amount of cholesterol is required in cells, on decrease of intracellular cholesterol, hepatocytes increase the expression of LDL receptors which then promote the extraction of LDL cholesterol from plasma secondarily. Are also known to influence LPL, especially in patients with Type 2 diabetes [138, 139]. These effects of statins occurred in a tissue specific manner, with an increased LPL production observed in skeletal muscle [140] and a decrease in LPL mass reported in macrophages [141].